"If we don't secure this well, this could be one of the most significant oil spills in U.S. history," Rear Adm. Mary Landry told reporters during a news conference Tuesday afternoon.

Although the slick had spread to about 20 miles off the Louisiana coast, Landry said experts were predicting that it would not reach land anywhere around the Gulf Coast in the next three days.

But she said shutting down the flow of oil could take three months.

That's why she is considering setting fire to the oil, perhaps as early as today. "It's a tool in the tool kit," she explained.

If Landry decides to take that step, the oil would be trapped in special containment booms and set ablaze. A similar burn off the coast of Newfoundland in 1993 eliminated 50 to 99 percent of captured oil.

She acknowledged that there could be risks, including causing air pollution problems and endangering marine creatures.

The oil is coming from a pipe rising from the seabed nearly a mile underwater. So far, crews using robotic subs have been unable to activate a shutoff device at the head of the well. A kink in the pipe is keeping oil from flowing even more heavily.

As of Tuesday morning, oil that leaked from the rig site had spread over an area about 48 miles long and 80 miles wide at its widest. The borders of the spill were uneven, making it difficult to calculate how many square miles were covered.

BP, which was leasing the Deepwater Horizon rig, will begin drilling relief wells near the source of the leak by Thursday. However, spokesman Robert Wine said it will take up to three months to drill a relief well from another rig. The company is spending $6 million a day trying to halt the spill and clean it up, company officials said.

The company also is designing a dome that could be submerged over the leak to corral the oil and route it up to vessels to be collected. However, the dome is at least two weeks away from being ready, company officials said, and that method has never been used at such a great depth.

Information from the Associated Press and the New York Times was used in this report.